This invention relates to refrigerant compressors driven by an electrical motor, and more particularly relates to means for controlling a refrigerant compressor utilizing a two-speed electrical motor.
Refrigerant compressors utilizing a two-speed compressor motor have been devised in the past. One such arrangement is shown in U.S. Pat. No. 3,584,980 (Cawley et al. -- June 15, 1971). Although a two speed compressor results in improved performance, the applicants have discovered that such an arrangement requires precise control in order to prevent damage to the compressor motor during two speed operation.
Compressor motors adapted to operate from a three phase source of electrical power generally comprise at least one stator coil for each of the electrical phases. The applicants have found that each of these coils must be protected from thermal damage in order to ensure long motor life. Since thermal damage can occur rapidly under certain adverse operating conditions, the coils must be protected by sensitive temperature detectors capable of rapid response.
Although one temperature sensor placed adjacent one of the coils will tend to provide protection against thermal destruction due to a locked rotor or running overload, a single sensor or dual sensors will not give adequate primary and secondary single phasing protection. The applicants have discovered that protection is enhanced by providing a sensor for each of the phase windings or coils in a multiphase motor. These sensors provide an indication of the temperature adjacent each coil associated with a different phase of electrical power. If one of the sensors indicates an abnormally high temperature in an associated stator coil, control means operate a switch in a motor pilot circuit in order to remove all electrical power from the motor. In this manner, the apparatus provides primary and secondary single phasing protection, along with locked rotor and running overload protection.
Accordingly, it is an object of the present invention to provide a refrigerant compressor in which a motor adapted to operate on multiphase electrical power has a temperature sensor located adjacent each motor coil associated with a different phase of the electrical power, so that each coil in the motor is adequately protected against thermal damage.
It is still another object of the invention to provide control apparatus of the foregoing type in which a control device interrupts power to the motor if any one of the sensors detects an abnormally high temperature in any of the motor coils.
The applicants have also discovered that the control of a two-speed compressor motor requires careful design of the contactors used to switch from a high speed mode of operation to a low speed mode of operation and vice versa. In a compressor motor adapted to be operated from multiphase electrical power, multiple stator coils are frequently provided and must be interconnected in different ways in order to achieve different speeds of operation.
The applicants have discovered that the interconnection of the coils must be achieved prior to the application of electrical power to the coils in order to avoid power surges and thermal damage to the motor. In addition, the applicants have invented a unique system of electrically and mechanically interlocking the contactors required for two-speed operation in order to assure a failsafe system of operation.
Accordingly, it is another object of the present invention to provide control apparatus for a two-speed compressor motor in which the coils in the motor are interconnected before any power is applied to the coils.
It is still another object of the present invention to provide motor control apparatus of the foregoing type in which both mechanical electrical interlocks are operated by and simultaneously with the contactors controlling the two-speed operation of the motor.